Molds for casting molten metals comprise several mold members working together to define the internal and external shape of the casting. Such members include core members for forming and shaping interior cavities in the casting, as well as cope/drag/shell members for forming and shaping the exterior of the casting. Such mold members are made by (1) mixing sand with a binder, (2) introducing (e.g., blowing) the binder-sand mix into a mold containing a pattern for shaping the sand-binder mix to the desired shape, and (3) curing/hardening the binder in the pattern mold to fix the shape of the mold member. During casting, molten metal is poured into or around the mold member(s), and allowed to solidify. The internal cores are removed from the casting by (1) hammering, (2) shaking, (3) heating to oxidize and crumble the binder, or (4) combinations thereof.
A variety of polymers are commonly used as binders in the so-called “hot-box”, “warm-box”, and “no-bake” techniques for making such mold members, as is well known to those skilled in the foundry art. Gelatin (a biopolymer) is also used as a binder as taught by Siak et al. U.S. Pat. No. 5,320,157 and Siak et al. U.S. Pat. No. 5,582,231, which are herein incorporated by reference. Gelatin is desirable because it is water soluble, environmentally benign, and low cost. Moreover, less heat is required to break the bonds of the gelatin's protein structure and oxidize the binder than is required for the other polymer binders. As a result, cores made with gelatin break-down readily from the heat of the molten metal alone, and thereby permit ready removal of the core sand (e.g., by shaking) from the casting with a minimum of additional processing. Powdered oxidation-promoting catalysts (i.e. ferric oxide, ferric phosphate and/or ferric pyrophosphate) have been added to the gelatin-sand mix to promote breakdown of the gelatin binder at aluminum casting temperatures. Following casting, the core sand is baked to remove charred and uncharred gelatin and reconstituted by adding fresh gelatin, as well as additional powdered catalyst to make up for catalyst “fines” (small particles) lost in handling and processing the mix. This process requires monitoring of the Fe2O3 content of the sand to determine how much make-up Fe2O3 is required as well as measuring the needed amount and mixing with the sand. It would be desirable to simplify the reconstitution process by eliminating the need to have to determine how much catalyst is needed as well as have to handle and mix the catalyst powders with the binder-sand every time a new batch of sand is prepared.
Moreover, in the so-called “Lost Foam” process, a fugitive foam pattern (e.g. polystyrene) is submerged in a bed of loose sand. Molten metal (e.g. Al) is poured onto the foam, which causes the foam to liquefy/vaporize and escape into the bed of sand where some of it remains as a residue. The metal fills the cavity left by the vaporized foam. The sand is reclaimed by heating in air to a temperature of about 760° C. for about one hour to oxidize and remove the residue from the sand. It would be desirable if this reclamation process could be accomplished quicker and at a lower temperature.
The present invention contemplates foundry sand modified to (1) accelerate the thermal oxidation of polymer binders or residues in foundry sand for more effective break-down thereof, and/or (2) eliminate the need to have to add fresh catalyst powders to each batch of sand-binder mix. With the modified sand, cores can be readily removed from castings, and polymer residues readily removed from sand during reclamation thereof.